The present invention relates to a thin film for recording information and its information recording medium and information recording/reproducing apparatus. In particular, the present invention relates to an information recording medium and an information memory apparatus capable of recording/reproducing on real time basis by using an energy beam such as laser light or an electron beam, information obtained by applying FM modulation to analog signals such as video signals or audio signals or digital information such as computer data, facsimile signals, or digital audio signals.
Various principles for recording information on a thin film (recording film) by exposing it to a laser beam are known. Among them, schemes using an atom configuration change caused by radiation of a laser beam, such as phase transition (also called phase change) of a film material or photodarkening, do not cause deformation of the thin film. Therefore, these schemes have the advantage that an information recording medium having a double side disk structure is obtained by sticking two sheets of disk member together directly. Furthermore, a GeSbTe recording film has the advantage of being capable of rewriting information.
In recording films of this kind, however, rewriting a large number of times exceeding 10.sup.4 times with a sample servo scheme or mark edge recording for attaining a higher density changes the thickness of the recording film due to a flow of the recording film and distorts the reproduced signal waveform. The flow of the recording film is caused as described below. Laser radiation in recording operation causes a flow of the recording film. By deformation caused by thermal expansion of a protective layer or an intermediate layer, the recording film is pushed little by little.
In "Optical Data Storage," by T. Ohta et al., '89 Proc. SPIE, 1078, 27 (1989), for example, there is disclosed a method of preventing the flow of a recording film by making the recording film thinner to reduce the heat capacity and increase the influence of adhesive power with respect to an adjacent layer. In 28P-L-11, 41st Joint Convention Record of Applied Physics, by Hirotsune, Terao, Miyauchi, Minemura, and Fushimi, p. 996, there is disclosed a method of preventing the flow of a recoridng film by adding a component having a high melting point to the recording film. Thereby, a great flow of the recording film can be suppressed. If rewriting is furthermore repeated a large number of times, however, a change in reflectance level is caused. Furthermore, in order to make the overwrite jitter characteristic in mark edge recording favorable, a disk increased in transmitted light is disclosed in "Relation between mark edge recording characteristics and thermal properties of phase change media," by Okubo, Murahata, Ide, Okada and Iwanaga, Convention Record of Fifth Meeting of Technical Group on Phase Change Recording, p.98. This disk structure is represented as PC substrate/ZnS--SiO.sub.2 (250 nm)/ Ge.sub.2 Sb.sub.2 Te.sub.5 (15 nm)/ZnS--SiO.sub.2 (18 nm)/Si (65 nm).
On some optical disks, digital information signals such as analog information signals obtained by applying FM modulation to video signals or audio signals or digital information signals such as computer data, facsimile signals or digital audio signals are transcribed as unevenness on the surface of the substrate. Some optical disks have information recording thin films capable of recording signals or data on the real time basis by using a recording beam such as a laser beam or an electron beam.. In these optical disks, the signal reproducing resolution is almost determined by the wavelength .lambda. of the light source of the reproducing optical system and the numerical aperture NA of the object lens, and the recording mark period 2NA/.lambda. is a read limit.
As for the technique for attaining a higher recording density, a method and a medium for reproducing data recorded in the form of unevenness by using a medium having a reflectance changed by a phase change are described in "Premastered Optical Disk by Superresolution," by K. Yasuda,. M. Ono, K. Aritani, A. Fukumoto, M. Kaneko, Jpn. Appl. Phys., vol. 32 (1993), pp. 5210-5213. In this technique as well, reading a large number of times exceeding 10.sup.4 times causes a flow in a film for reading with superresolution and hence a change in reflectance level, the number of times of allowed reading operations being thus limited.
Herein, the term "phase change" includes not only a phase change between a crystalline state and an amorphous state, but also fusion (a change to a liquid phase), recrystallization, and a change between a crystalline state and another crystalline state.
Any information recording medium having a conventional recording film has a problem that increasing the number of times of allowed rewriting operations causes a change in reflectance level in the case where the medium is used as a rewritable information recording medium of phase transition type.
In the same way, any information recording medium having a conventional superresolution reading film has a problem that increasing the number of times of allowed reading operations with a superresolution causes a change in reflectance level in the case where the medium is used as a superresolution readable information recording medium of phase transition type.